1. Field of the Invention
The present invention relates to a display device and a method of driving the same. The present invention relates particularly to an active matrix type display device having a thin film transistor (referred to as a TFT hereinafter) formed on an insulator, and a method of driving the same, more particularly to an active matrix type display device using digital signals as picture signals, and a method of driving the same.
2. Related Art
In recent years, display devices having elements formed using a semiconductor thin film on an insulator, particularly on a glass substrate, have been spreading. For example, active matrix type display devices using a TFT have been spreading. In an active matrix display device, pixels are arranged in a matrix form and TFTs are arranged onto the respective pixels (the TFTs being referred to as pixel TFTs hereinafter). The pixel TFTs are used to control the brightness of the respective pixels, thereby displaying images.
Recently, there has been developing a technique of using a polycrystal semiconductor to form not only pixel TFTs constituting pixels but also TFTs constituting a driving circuit at the same time. This technique contributes greatly to miniaturization and low power consumption of display devices. Following this, an active matrix type display device has been becoming essential for the display section of portable information-processing equipment, the applicable field of which has been markedly expanding in recent years, or the like section. Examples of the active matrix type display device include an active matrix liquid crystal display device using a liquid crystal element, and an active matrix type organic light emitting diode (OLED) display device using an OLED element. In the present specification, attention is paid mainly to the active matrix type liquid crystal display device.
The above-mentioned liquid crystal element is composed of two electrodes, oriented films formed on the respective electrodes, and a liquid crystal material sandwiched between faces of the two electrodes on which the respective oriented films are formed. As the liquid crystal material, any material having a known structure can be used.
FIG. 6 schematically illustrates a conventional active matrix type liquid crystal display device of a system in which digital picture signals are used to perform display (referred to as a digital system herein after). At the center thereof, a pixel section 1308 is arranged.
In the pixel section 1308, plural pixels are arranged in a matrix form. Plural source signal lines and plural gate signal lines for inputting signals into the respective pixels are arranged.
A source signal line driving circuit 1301 for controlling signals to be inputted into the source signal lines is arranged over the pixel section 1308.
The source signal line driving circuit 1301 has a shift register 1303, the first latch circuit 1304, the second latch circuit 1305, D/A (digital/analogue) converter circuit 1306, which is illustrated as DAC in FIG. 6, an analogue switch 1307, and so on. Gate signal line driving circuits 1302 for controlling signals to be inputted to gate signal lines are arranged at the right and left sides of the pixel section 1308. Only one gate signal line driving circuit 1302 may be arranged at one side of the pixel section 1308. However, the case in which the gate signal line driving circuits are arranged at both the sides of the pixel section 1308 is more preferred from the viewpoints of driving efficiency and driving reliability.
The source signal line driving circuit 1301 has a configuration as illustrated in FIG. 7. The source signal line driving circuit, the example of which is illustrated in FIG. 7, is a source signal line driving circuit corresponding to a display device which has pixels, the number of which is x in the horizontal direction, so as to display gradation by the input of 3-bit digital picture signals (the gradation being referred to as 3-bit digital gradation).
The source signal line driving circuit illustrated in FIG. 7 has a shift register circuit (SR) 1401, the first latch circuit (LAT1) 1402, the second latch circuit (LAT2) 1403, D/A converter circuit (DAC) 1404, and so on. In FIG. 7, the analogue switch 1307 illustrated in FIG. 6 is not illustrated. If necessary, a buffer circuit, a level shift circuit, and so on, which are not illustrated in FIG. 7, may be arranged.
Referring to FIGS. 6 and 7, the following will describe the operation of the display device. First, clock signals (clock pulses, inverting clock pulses) and a start pulse are inputted to the shift register 1303, which are represented by “SR” in FIG. 7. As a result, pulses are successively inputted from the shift register circuit 1303 to the first latch circuit 1304, which are represented by “LAT1” in FIG. 7, so as to hold digital picture signals (digital data) which are similarly inputted to the first latch circuit 1304.
The most significant bit (MSB) of the digital picture, signals is represented by D3, and the least significant bit (LSB) of the digital picture signals is represented by D1. After the holding of the digital data corresponding to one horizontal term is completed in the first latch circuit 1304, during a retrace line period the digital picture signals held in the first latch circuit 1304 are simultaneously transferred to the second latch circuit 1305, which is represented by “LAT2” in FIG. 7, by the input of a latch signal (latch pulse).
Thereafter, the shift register circuit 1303 is again operated to start the holding of digital data corresponding to the next horizontal term. At the same time, the digital data held in the second latch circuit 1305 are converted to analogue signals in the D/A converter circuit 1306, which is represented by “DAC” in FIG. 7. The analogue signals are inputted to the source signal lines, represented by “S1” to “Sx” in FIG. 7, and written in the respective pixels.
FIG. 8 illustrates a configuration of the pixel section of an ordinary active matrix type liquid crystal display device.
In each of pixels, a condenser 1001, a switching TFT 1002, and a liquid crystal element 1003 are arranged. The gate electrode of the switching TFT 1002 in each of the pixels is connected to some line of the gate signal lines G1 to Gy. One of the source region and the drain region of the switching TFT 1002 in each of the pixels is connected to some line of the source signal lines S1 to Sx, and the other is connected to either electrode of the condenser 1001 and either electrode of the liquid crystal element 1003.
The analogue signals inputted to the source signal lines S1 to Sx are inputted to the condensers 1001 and the liquid crystal elements 1003 across the drain and the source of the switching TFTs 1002 which have become conductive by the signals inputted to the gate signal lines G1 to Gy. Depending on the voltages of the signals, the transmittivity of the liquid crystal elements 1003 varies so that the brightness of the respective pixels is represented.
When an electric field along a given direction is constantly applied between the two electrodes of the liquid crystal element, ions in the liquid crystal material are prejudiced, thereby resulting in a problem that the liquid crystal element deteriorates. Thus, in display devices or the like wherein the ordinary liquid crystal element is used, there is used a driving method of changing, at regular intervals, the polarity of the voltage applied to the liquid crystal element so as to change the direction of the electric field applied to the two electrodes of the liquid crystal element.
For example, the following driving methods are known: a driving method called gate line inversion, a driving method called source line inversion, and a driving method called frame inversion.
In the driving method called gate line inversion, the polarities of signal voltages applied to liquid crystal elements are made different between gate signal lines adjacent to each other. In the driving method called source line inversion, the polarities of signal voltages applied to liquid crystal elements are made different between source signal lines adjacent to each other. In the driving method called frame inversion, the polarity of the signal voltage applied to the liquid crystal is inverted in every period when an image corresponding to one frame is displayed (the period being referred to as a frame period hereinafter).
Referring to timing charts shown in FIGS. 8 and 9, the following will describe the operation of this conventional active matrix type liquid crystal display device.
About the timing chart shown in FIG. 9, an operation based on the frame inversion driving is used.
Signals having a polarity contrary to signals inputted to the source signal line in the first frame period (F1) are inputted from the source signal line in the second frame period (F2). In the third frame period (F3), signals having a polarity different from that of the signals inputted in the second frame period (F2) are inputted.
In the first frame period (F1), the gate signal line G1 is firstly selected. As a result thereof, the switching TFT 1002 whose gate electrode is connected to the gate signal line G1 conducts. Thereafter, signals are inputted through the source signal lines S1 to Sx.
In the timing chart of FIG. 9, attention is paid to a certain source signal line Sm (m is a natural number of x or less) and only signals inputted to this source signal line Sm are shown. The period during which one gate signal line is selected is referred to as one horizontal term (one line period: L). Particularly, the period during which the gate signal line G1 is selected is referred to as the first line period L1.
After the input of a signal to the pixels having the switching TFTs 1002 connected to the gate signal line G1 finishes, a signal is inputted to the gate signal line G2 so that all of the switching TFTs 1002 connected to the gate signal line G2 conduct. In this way, the input of signals in the second line period L2 starts.
The above-mentioned operation is repeated about all of the gate signal lines G1 to Gy so that the repeated operation finishes in the yth line period Ly. As a result, one frame period ends.
Next, the second frame period (F2) starts. In the second frame period (F2), the polarity of signals inputted to the source signal line is different from the polarity of the signal voltage, of the source signal line, inputted to the source signal line in the first frame period (F1). In this way, images are displayed.
After the second frame period (F2) finishes, the third frame period (F3) starts. In the third frame period (F3), signal voltage having a polarity different from that of the signal voltage in the second frame period (F2) is inputted to the source signal line. In other words, signal voltage having the same polarity as in the first frame period is inputted to the source signal line.
The above-mentioned operation is repeated to display images.
In an ordinary active matrix type liquid crystal display device, display in its screen is renewed about 60 times per second in order to make the display of moving images smooth. In other words, it is necessary to supply digital picture signals in every frame period by the above-mentioned operation and perform writing in all of the pixels every time. Even if the picture to be displayed is a still image, the same signals must be continuously supplied in every frame period. It is therefore necessary that an external circuit, the driving circuit and so on continuously perform repetitive processing of the same digital picture signals.
There is also known a method of writing digital picture signals for a still image once in an external memory circuit and subsequently supplying the digital picture signals from the external memory circuit to a liquid crystal display device in every frame period. In either case, it is necessary that the external memory circuit and the driving circuit operate continuously.
Particularly in portable information-processing equipment, it is desired to make the power consumption thereof low. In portable information-processing equipment, the period during which a still image is continuously displayed occupies most of all periods. Notwithstanding this fact, the external circuit, the driving circuit and so on must operate continuously at the time when the still image is displayed, as described above. This fact prevents the power consumption from being made low.